1. Field of the Invention
The present invention relates to a photothermal transducing type of recording medium for recording information with high density and reproducing the recorded information by means of a laser beam or the like, wherein the photothermal transducing effect is utilized. More particularly, the invention relates to a photothermal transducing recording medium capable of effectively absorbing light of wavelength in the visible or near infrared region and transducing its energy to thermal energy, and further performing the recording with high density and optical reproduction of the recorded information by means of a laser beam or the like.
2. Description of the Prior Art
In photothermal transducing recording media used in the optical disk technique, high-density information can be recorded by forming spiral or circular tracks of minute (for instance, about 1.mu.) optically detectable pits in their thin photothermal transducing recording layers laid on substrates. For writing information in such an optical disk, the laser-sensitive layer surface thereof is spirally or circularly scanned with a converged laser beam to form pits only at the portions irradiated with the laser beam. The pits may be in the spiral or circular track form. The laser-sensitive layer can absorb energy of the laser beam, forming optically detectable pits. According to, for example, the heat-mode recording system, the laser-sensitive layer absorbs energy of the incident laser beam and converts it into thermal energy to form minute recesses (pits) at the irradiated positions through evaporation or deformation of these portions thereof or to form minute pits there which show optically detectable difference, due to a chemical change, in oxidation degree, reflectivity, or optical density from the nonirradiated portions.
The information recorded in the optical disk is read by allowing a laser beam to scan along the track and detecting optical change or difference between the portion where the pit is formed and the portion where no pit is formed. For instance, the track is scanned with a laser beam, and the energy reflected from the disk is monitored with a photodetector. Output from the photodetector will be reduced when no pit is formed, and will be increased when a pit is formed, since the beam is sufficiently reflected from the reflecting interface of the underlying layer.
For the recording media used in such optical disks, there have been proposed chiefly the use of inorganic materials such as metal films such as aluminum films vapor-deposited, bismuth thin films, tellurium oxide thin films, and chalcogenide type amorphous glass films.
On the other hand, liquid crystal devices according to photothermal transducing recording techniques are capable of forming optical images in response to light signals supplied from a laser or the like. The hitherto used liquid crystal device of this type has a mixture of a nematic liquid crystal with negative dielectric anisotropy and a cholesteric liquid crystal between two glass base plates or a smectic liquid crystal with positive dielectric an isotropy therebetween. On irradiating this liquid crystal device with a laser beam or the like, thermal energy evolves in the irradiated regions so that isotropic phase is produced by heating. Thereafter, by rapid cooling, the liquid crystal phase in the random orientation state of the liquid crystal molecules is formed which is dissimilar to the initial uniform orientation state. Thus, the region irradiated with the laser beam can now scatter incident light so that difference in optical properties is produced between the irradiated region and the background region where the liquid crystal molecules are uniformly oriented.
This type of liquid crystal device permits also erasing the optical image formed therein by laser beam writing as described above. That is, an electrode is formed on each of the two base plates constructing the liquid crystal device, and this liquid crystal device is entirely heated with a heat source (e.g. an electric heater) other than the laser beam, thereby turning the liquid crystal phase to the isotropic phase. The device is cooled until, for example, a homeotropic structure is formed in case of the smectic liquid crystal, or a grandjean structure is formed in case of the cholestericnematic liquid crystal, whereby the previously written optical image can be erased.
Such a liquid crystal device based on a photothermal transducing recording system is advantageous in that an image can be formed simply by scanning the liquid crystal surface with light signals converted from electric signals without requiring the matrix electrode structure to form picture elements, and that the image can be obtained on a large scale. However, this type of device has a drawback that the efficiency of absorbing and converting the laser beam energy, when it is used, into thermal energy is insufficient and therefore satisfactory writing cannot be effected by the scanning with light signals. Accordingly, a guest-host type of photothermal transducing recording liquid crystal device which comprises a smectic liquid crystal containing a black colorant has been proposed as disclosed, for example, in "Society of Information Display International Symposium, Digest of Technical Paper", pp. 34-49, 172-187, 238-253 (1982).
Meanwhile, there have been developed in recent years small-sized, low cost semiconductor lasers which are capable of being modulated directly. Many of these lasers have wavelengths of at least 700 nm and generally the powers of the laser beams are low as compared with those of gas lasers such as argon lasers and helium-neon lasers. Accordingly, when photothermal transducing recording is carried out with such a semiconductor laser, it is effective to use a laser-sensitive layer having an absorption maximum in a longer-wavelength region (generally the region of 700-850 nm).
The conventional photothermal transducing recording media, however, are not sufficient in the efficiency of absorbing a laser beam and transducing its energy to thermal energy. In case of optical disks, as an example, a photothermal transducing recording layer composed mainly of an inorganic material as mentioned above has a high reflectivity for laser beams. Such a recording layer suffers from disadvantages in that the efficiency of laser energy utilization is low and therefore high sensitivity characteristic cannot be obtained. Also, the laser-sensitive layer requires a complicated structure in order to make the layer sensitive to rays of wavelengths of at least 700 nm. In view of the above, there have been studied in recent years organic compounds the physical properties of which can be changed by the energy of light in relatively long wavelength region. It is known that, for example, pyrylium dyes as disclosed in U.S. Pat. No. 4,315,983, "Research Disclosure", 20517 (1981, 5) and squarium dyes as disclosed in "J. Vac. Sci. Technol.", 18(1), pp. 105-109 (Jan./Feb. 1981) are sensitive to laser of wavelength of at least 700 nm.
However, there are problems such that organic compounds having absorption maxima in the longer wavelength region are generally more unstable and liable to decompose with a slight rise in temperature.
The guest-host type of photothermal transducing recording liquid crystal device also has a drawback that when a semiconductor laser is used, the efficiency of absorbing and transducing laser beam energy to thermal energy is insufficient since the output power of the laser is low and it is therefore necessary to scan the surface with light signals of high power or low speed. The black colorant-containing liquid crystal device mentioned above has the drawback that the display is unfavorable in the aspect of human engineering since a white image pattern is formed in the black background.
Therefore, it cannot be said that photothermal transducing recording media hitherto developed for optical disk or liquid crystal device applications are sufficiently satisfactory for practical use since these media need to fulfill requirements in various properties.